As is known, large marine diesel engines use a heavy oil referred to as heavy fuel oil (HFO) as fuel. This oil is constituted by a mixture of paraffins and aliphatic hydrocarbons and with a high sulphur content (1%-3.5%)
The use of this fuel is substantially imposed on large ships for economic reasons, in that being an unrefined fuel and being of a lower quality with respect to the fuel used in diesel engines for automotive use, it an be purchased at a markedly lower cost. However, it also entails significant problems in terms of environmental pollution due to the pollutant content (sulphur, ashes, heavy metals, etc.)
According to what is known, the combustion of heavy oil generates high particulate loading (liquid, semi-liquid and solid microparticles) in the exhaust gases. The particulate can be constituted by unburned hydrocarbons which form the so-called soot, and/or by impurities, even inorganic impurities, initially present in the fuel, which form the ashes, combustion residues.
In order to prevent this particulate from being released into the atmosphere, particulate filters are employed in the automotive sector, which are able to retain the solid particulate by trapping it in the filter. The agglomerates tend to clog the filter, which generally has a construction of the honeycomb type, and it therefore cyclically necessary to provide for the regeneration of the filter itself, generally by proceeding with the further combustion of the agglomerates.
In the marine sector, namely for the large diesel engines with which ships are equipped, on the other hand, there is not currently any dry filtration of the particulate as the high content of impurities in the naphthas used makes the use of particulate filters of the type currently used in the automotive sector substantially impossible, as they would immediately clog due to the high presence of non-combustible particulate in the exhaust fumes. In addition, these particulate filters, which generally use oxidising catalysts to improve oxidation of the organic fraction, and thus the combustion of soot, in the event of these being used in marine engines that burn diesel fuel containing sulphur, would result in oxidation of the SO2 to SO3, further increasing the particulate to an excessive extent. SO3 is a compound, which condenses in the presence of water vapour and temperatures below 200° C., forming a sulphuric acid aerosol. Once emitted to the funnel, this aerosol produces a very visible brownish-blue plume that in fact increases the total particulate emission (liquid and semi-liquid) with respect to the case of the absence of the particulate filter. The micro and sub-micron dimensions of this aerosol prevent its removal by means of a possible seawater scrubber present in the exhaust system. Thus, no dry filtering system of the exhaust gases of marine diesel engines is currently used, while a “wet” filtering system is used to eliminate the particulate and the SO2 namely by means of the use of countercurrent “scrubbers” that use seawater directly, in accordance with the teachings of the prior art, to eliminate the particulate material and absorb the acid pollutants.
The efficacy of the “scrubber” on the particulate matter is however somewhat limited and is also limited to transferring the particulate from the gaseous phase to the liquid phase polluting the seawater used therein. The seawater leaving the “scrubber”, which is dispersed in the sea, contains, oils, polycyclic aromatic hydrocarbons—also known by the acronym PAH—which have known adverse effects on the environment (evident toxicity to certain aquatic organisms and birds, high chronic toxicity to aquatic life, contamination of agricultural crops), and on human health, heavy metals, such as vanadium, chromium, cadmium, nickel and many more, as well as ashes.